Radio Frequency Identification (RFID) tags are becoming increasingly common. RFID tags that include sensing capabilities have emerged as a generally inexpensive and effective means of addressing many wireless sensor applications in both indoor and outdoor sensing applications. Purely passive sensors, such as RFID tags, when actively interrogated by an RF transceiver/reader, receive energy to power themselves up so that they can acquire readings from their attached sensing elements. Generally, RFID tags equipped with one or more sensors require a source of energy to measure and store their acquired information at times other than during active interrogation by a reader. Standard passive (battery-less) RFID tags provide no means of acquiring sensor information when they are outside the communication range of a reader.
Next generation sensor networks may be powered by energy harvesting techniques to avoid requiring battery maintenance. Energy harvesting is a process by which energy is derived from external sources (e.g., radio frequency energy, solar power, thermal energy, wind energy, salinity gradients, or kinetic energy), captured and stored.
Energy may be harvested from radio frequency signals propagating wirelessly. With RF harvesting, wireless energy comes from a radio frequency transmitting device that is some distance away from a device that harvests energy from the radio frequency transmission.
One of the more popular forms of RF used today is Wi-Fi (also referred to as IEEE 802.11a/b/g/n etc.) communications. Today, most Wi-Fi communications are in the 2.4 GHz and 5.8 GHz frequency bands and there are many local area networks that are based on Wi-Fi in which access points enable Wi-Fi clients to gain access to networks such as the Internet. Furthermore, the 2.4 GHz and 5.8 GHz bands also support other networking standards, such as Zigbee and Bluetooth, and other proprietary networks, each transmitting energy by communicating in this same frequency band. Additionally there are other frequency bands that support different communication protocols, each of which transmit energy when they are communicating.
In a traditional RFID-like setup the RFID readers (interrogators) usually deliver RF power as a continuous wave (CW), i.e. a sinusoidal signal of a particular frequency. In a traditional system with several readers, a combination of time-sharing between the readers, and a defined frequency allocation plan is used to increase information throughput of the system as a whole. In addition to its spectrum sharing advantages, frequency planning/frequency hopping is also an important mechanism used to combat the effects of frequency-dependent fading in the typical multipath environment. If a transponder is in a particular location exhibiting a deep RF fade (sometimes called a null) at one frequency, a relatively small change in frequency is likely to move it out of the RF fade.
Increasing the amount of incident power at transponder is an important problem, as it typically leads to the increase in both the harvestable energy, and the energy available for communications (backscatter). However, the transmit power of the interrogators (readers), is ultimately limited because of regulations imposed by regulatory bodies (the FCC in case of the USA), capping the emitted RF power at certain levels specific to each RF band. Also, increasing the transmit power is not the most efficient approach in case of multipath environment, or in face of a highly nonlinear behavior of the harvester element (such as one or more diodes or diode-connected transistors) of the transponder device, as it precludes selective manipulation of power delivered to transponders. Ultimately one may want to boost energy flow to some transponders, while starving some other transponders, where this energy allocation may be adjusted over time according to certain schedule.
In attempt to maximize the efficiency of diode-based RF energy harvesters, Durgin et al., described the use of power-optimized waveforms (POWs). In the POW method one replaces the traditional single-frequency continuous wave (CW) RF signal with a particular complex waveform comprised of multiple frequency components (subcarriers), optimized for a particular RF energy harvester circuit (such as a rectenna with voltage boost circuit) to provide the maximum amount of harvestable energy at a particular average power level. POWs take advantage of highly nonlinear behavior of rectifier element in harvesters at low incident power levels, by “squeezing” energy in shorter bursts to maximize the peak RF voltage, while holding the average power at the same level. This technique results in significant boost in the harvester efficiency, especially pronounced at low incident power levels close to the harvesting threshold—see FIG. 2b. Note, that for a given set of tunable elements, a POW is a function of particular harvester design. For more information, see US20110148221 (Trotter M.; Durgin G., GTRC), “Systems and Methods for Providing a Power Optimized Waveform,” and Valenta, C.; Durgin, G. “Rectenna Performance Under Power-optimized Waveform Excitation,” IEEE RFID Conference, 2013.
To overcome the effects of frequency selective fading, and to potentially derive an advantage from multipath RF environments, a Wireless Power Transfer (WPT) Optimization method proposed by D. Arnitz and M. Reynolds. This method consists of a MIMO base station, and a procedure to control the base station to selectively minimize or maximize incident power at transponders. They used measurements of backscatter signal amplitude and phase as received by the MIMO base station receivers (RX), as a proxy for the incident power at the transponder, and they have proven that under certain assumptions the problem of maximizing a weighted RF power at the MIMO base station receivers is equivalent to that of maximizing the incident power at the transponder. In the MIMO WPT optimization method the optimization parameters are amplitudes and phases of subcarriers at MIMO base station transmitters and/or transmitter (TX) antennas. For more information on the MIMO WPT Optimization method, see Arnitz, D.; Reynolds, M. S., “Wireless Power Transfer Optimization for Nonlinear Passive Backscatter Devices,” IEEE RFID Conference, 2013 and Arnitz, D.; Reynolds M. S., “Multitransmitter Wireless Power Transfer Optimization for Backscatter RFID Transponders,” IEEE Antennas and Wireless Propagation Letters, vol. 12, no., pp. 849-852, 2013 doi: 10.1109/LAWP.2013.2271984.